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Tile floor, drywall on all interior walls except brick wall. Southwest exterior is brick. No trees/shade. Black asphalt roof with ridge vent and soffit venting (and gable vent that was never sealed ....) Truss attic with 22"x22" access inside of a closet at edge of room.

Current system: Trane 1.5 ton heat pump mounted in attic. 5 supply registers in ceiling with 1 return in wall with filter. System had condensor replaced about 10 years ago (Trane XE1000 replaced original XE800). Evaporator coils were not replaced. Not sure about duct-work condition (was installed 20 years ago by one of the listed/recommended contractors here).

Challenge with replacing system (if we decide to do so):
Unit installed before drywall installed - will need to cut large hole in drywall to replace existing unit in attic. (Not a big deal)
Original contractor installed 1 supply to garage (Big no-no - must be removed ASAP no matter what .... CO poisioning issue ...)

Options:
1) Repair unit (system cools marginally now, heated marginally last winter) - current system was fine for 18+ years .... HVAC contractor did not test condensor yet .... just diagnosed constant fan and no backup heat problem.
2) Replace unit with modern similar split system (would need to do some major drywall removal, not a huge issue) - manual-j would be requested (do they sell smaller than 1.5 ton??) Current HVAC contractor installed Carrier Performance system in main house, would inquire about Carrier Greenspeed for this room.
3) Abandon current system and install mini-split in room of proper size. Might desire having ceiling-mounted system verus wall mounted system due to low ceilings and little room above windows/doors for wall-mounted system. Current HVAC contractor does not have mini-split experience and expressed concerns about even heating/cooling from one unit mounted on wall (was not aware of ceiling mount units).

While I do not live in your area now... I lived in Falls Church a few decades ago... kinda remember the climate.

The mini will use less electricity and feel a bit better in the winter. Air circulation will be a bit of an issue in a room that large with one ceiling cassette mini... and not all companies make one head mini systems using the ceiling cassette.

Here is a different solution that takes advantage of both worlds... albeit find a heat and AC company that has experience with it:

Tile floor, drywall on all interior walls except brick wall. Southwest exterior is brick. No trees/shade. Black asphalt roof with ridge vent and soffit venting (and gable vent that was never sealed ....) Truss attic with 22"x22" access inside of a closet at edge of room.

Current system: Trane 1.5 ton heat pump mounted in attic. 5 supply registers in ceiling with 1 return in wall with filter. System had condensor replaced about 10 years ago (Trane XE1000 replaced original XE800). Evaporator coils were not replaced. Not sure about duct-work condition (was installed 20 years ago by one of the listed/recommended contractors here).

Challenge with replacing system (if we decide to do so):
Unit installed before drywall installed - will need to cut large hole in drywall to replace existing unit in attic. (Not a big deal)
Original contractor installed 1 supply to garage (Big no-no - must be removed ASAP no matter what .... CO poisioning issue ...)

Options:
1) Repair unit (system cools marginally now, heated marginally last winter) - current system was fine for 18+ years .... HVAC contractor did not test condensor yet .... just diagnosed constant fan and no backup heat problem.
2) Replace unit with modern similar split system (would need to do some major drywall removal, not a huge issue) - manual-j would be requested (do they sell smaller than 1.5 ton??) Current HVAC contractor installed Carrier Performance system in main house, would inquire about Carrier Greenspeed for this room.
3) Abandon current system and install mini-split in room of proper size. Might desire having ceiling-mounted system verus wall mounted system due to low ceilings and little room above windows/doors for wall-mounted system. Current HVAC contractor does not have mini-split experience and expressed concerns about even heating/cooling from one unit mounted on wall (was not aware of ceiling mount units).

My questions to the pros:
1) Seems a mini-split might be very effecient for a single open room of this size?
2) Should we put money into current old system?
3) Other ideas?

1). Yes. mini-splits can get up to 21 SEER and this is a lot cheaper than what you would be paying for a similar efficiency split system. Mini-splits also get as small as 1 ton, smallest split systems are around 1.5 to 2 tons.
2). I know some units have lasted longer, but usually it is around 15 years before major repairs. Your repairs don't seem that extensive yet, but the perk is you will be saving on your electricity bill in addition to not paying for repair cost.
3. Ceiling mounted systems are available by all manufacturers... LG, Mitsubishi, Samsung, Fujitsu, etc... If it is only serving one room you should be fine. If you really want, the mini-splits come in multi-zone models (a lot more expensive) that allows you to split the cooling between two or more indoor units.

You can call me Sam

It should be a crime to be a mechanical engineer in San Diego
Summer Design Temperature: 83 F Dry Bulb ~ 69 F Wet Bulb (California Climate Zone 7)